Prior to the present invention, the liquid spray application of coatings, such as lacquers, enamels and varnishes, was effected solely through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution resulting from painting and finishing operations. For this reason there has been a great deal of emphasis placed on the development of new coatings technologies which diminish the emission of organic solvent vapors. A number of technologies have emerged as having met most but not all of the performance and application requirements, and at the same time meeting emission requirements and regulations. They are: (a) powder coatings, (b) water-borne dispersions, (c) water-borne solutions, (d) non-aqueous dispersions, and (e) high solids coatings. Each of these technologies has been employed in certain applications and each has found a niche in a particular industry. However, at the present time, none has provided the performance and application properties that were initially expected.
Powder coatings, for example, while providing ultra low emission of organic vapors, are characterized by poor gloss or good gloss with heavy orange peel, poor distinctness of image gloss (DOI), and poor film uniformity. Moreover, to obtain even these limited performance properties generally requires excessive film thicknesses and/or high curing temperatures. Pigmentation of powder coatings is often difficult, requiring at times milling and extrusion of the polymer-pigment composite mixture followed by cryogenic grinding. In addition, changing colors of the coating often requires its complete cleaning, because of dust contamination of the application equipment and finishing area.
Water-borne coatings are very difficult to apply under conditions of high relative humidity without serious coating defects. These defects result from the fact that under conditions of high humidity, water evaporates more slowly than the organic cosolvents of the coalescing aid, and as might be expected in the case of aqueous dispersions, the loss of the organic cosolvent/coalescing aid interferes with film formation. Poor gloss, poor uniformity, and pin holes unfortunately often result. Additionally, water-borne coatings are not as resistant to corrosive environments as are the more conventional solvent borne coatings.
Coatings applied with organic solvents at high solids levels avoid many of the pitfalls of powder and water-borne coatings. However, in these systems the molecular weight of the polymer has been decreased and reactive functionality has been incorporated therein so that further polymerization and crosslinking can take place after the coating has been applied. It has been hoped that this type of coating will meet the ever-increasing regulatory requirements and yet meet the most exacting coatings performance demands. However, there is a limit as to the ability of this technology to meet the performance requirement of a commercial coating operation. Present high solids systems have difficulty in application to vertical surfaces without running and sagging of the coating. Often, they are also prone to cratering and pin holing of the coating. If they possess good reactivity, they often have poor shelf and pot life. However, if they have adequate shelf stability, they cure and/or crosslink slowly or require high temperature to effect an adequate coating on the substrate.
Clearly, what is needed is an environmentally safe, non-polluting diluent that can be used to thin very highly viscous polymer and coatings compositions to liquid spray application consistency. Such a diluent would allow utilization of the best aspects of organic solvent borne coatings applications and performance while reducing the environmental concerns to an acceptable level. Such a coating system could meet the requirements of shop- and field-applied liquid spray coatings as well as factory-applied finishes and still be in compliance with environmental regulations.
Such a needed diluent has now been found and is discussed in the aforementioned related applications which teach, among other things, the utilization of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute these compositions to application viscosity required for liquid spray techniques.
U.S. patent application Ser. No. 133,068, filed Dec. 21, 1987, to Hoy, et al., disclose processes and apparatus for the liquid spray application of coatings to a substrate that minimize the use of environmentally undesirable organic diluents. The broadest process embodiment of that application involves:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA1 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon. PA1 (1) means for supplying at least one polymeric compound capable of forming a continuous, adherent coating; PA1 (2) means for supplying at least one active organic solvent; PA1 (3) means for supplying supercritical carbon dioxide fluid; PA1 (4) means for forming a liquid mixture of components supplied from (1)-(3); and PA1 (5) means for spraying said liquid mixture onto a substrate. PA1 (1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA1 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray. PA1 (1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA1 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray; and PA1 (3) electrically charging said liquid spray by a high electrical voltage relative to the substrate and electric current. PA1 (a) be particularly compatible for subsequent admixture with a supercritical fluid diluent; PA1 (b) be particularly suitable, once admixed with the supercritical fluid, to help minimize any of the phenomena that may occur which are peculiarly associated with the utilization of such supercritical fluid, which phenomena may interfere with proper atomization of the admixed liquid mixture and/or proper diffusion of the supercritical fluid once atomized; and PA1 (c) provide the necessary coating characteristics such that once sprayed onto a substrate, it will help provide the necessary coalescence of the deposited droplets to form a coherent liquid coating while still not causing sagging or runs and help minimize any of the other defects noted above while at the same time, still allow for the release of any residual supercritical fluid that may be present after the coating has been applied to the substrate. PA1 (a) a solids fraction containing at least one polymeric compound capable of forming a coating on a substrate; and PA1 (b) a solvent fraction containing at least one coupling solvent in which said at least one polymeric compound is at least partially soluble and which is at least partially miscible with water; and PA1 (c) water, which is present in an amount of less than about 30% by weight based on the weight of the solvent fraction; PA1 said liquid mixture having: PA1 (a) forming a liquid mixture in a closed system, said liquid mixture comprising: PA1 (b) spraying said liquid mixture onto a substrate to form a liquid coating thereon.
(a) at least one polymeric compound capable of forming a coating on a substrate; and PA2 (b) at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture of (a) and (b) to a point suitable for spray application; and PA2 (a) at least one polymeric component capable of forming a coating on a substrate; and PA2 (b) a solvent component containing at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application; and PA2 (a) at least one polymeric component capable of forming a coating on a substrate; and PA2 (b) a solvent component containing at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application; PA2 (i) a viscosity of less than about 6,000 centipoise and having less than about 650 grams of the solvent fraction per liter of mixture; PA2 (ii) a solubility with at least one supercritical fluid, above the critical temperature and pressure of the supercritical fluid, of greater than 5% by weight of supercritical fluid in said mixture; PA2 (iii) a viscosity of less than about 300 centipoise when admixed with a sufficient amount of the at least one supercritical fluid, above the critical temperature and pressure of the supercritical fluid, so as to render the mixture suitable for spray application; and PA2 (iv) a solubility with the supercritical fluid in the non-supercritical state, at 25.degree. C. and one atmosphere absolute pressure of said fluid, of less than about 0.8% by weight of fluid in said mixture. PA2 (i) a solids fraction containing at least one polymeric compound capable of forming a coating on a substrate; PA2 (ii) a solvent fraction containing at least one coupling solvent in which said at least one polymeric compound is at least partially soluble and which is at least partially miscible with water; PA2 (iii) water, which is present in an amount of less than about 30% by weight based on the weight of the solvent fraction; and PA2 (iv) at least one supercritical fluid, in at least an amount which when added to (i), (ii), and (iii) is sufficient to render the viscosity of said mixture to a point suitable for spray application; and
That application is also directed to a liquid spray process in which at least one active organic solvent (c) is admixed with (a) and (b) above prior to the liquid spray application of the resulting mixture to a substrate. The preferred supercritical fluid is supercritical carbon dioxide. The process employs an apparatus in which the mixture of the components of the liquid spray mixture can be blended and sprayed onto an appropriate substrate. The apparatus contains
The apparatus may also provide for (6) means for heating any of said components and/or said liquid mixture of components. U.S. patent application Ser. No. 133,068 demonstrates the use of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute the compositions to application viscosity required for liquid spray techniques. It further demonstrates that the method is generally applicable to all organic solvent-borne coatings systems.
Copending U.S. application Ser. No. 218,910, filed Jul. 14, 1988, is directed to a liquid coatings application process and apparatus in which supercritical fluids, such as supercritical carbon dioxide fluid, are used to reduce to application consistency, viscous coatings compositions to allow for their application as liquid sprays. The coatings compositions are sprayed by passing the composition under pressure through an orifice into the environment of the substrate.
In particular, the process of U.S. application Ser. No. 218,910 for liquid spray application of coatings to a substrate comprises:
U.S. application Ser. No. 218,895, filed Jul. 14, 1988, is directed to a process and apparatus for coating substrates by a liquid spray in which 1) supercritical fluid, such as supercritical carbon dioxide fluid, is used as a viscosity reduction diluent for coating formulations, 2) the mixture of supercritical fluid and coating formulation is passed under pressure through an orifice into the environment of the substrate to form the liquid spray, and 3) the liquid spray is electrically charged by a high electrical voltage relative to the substrate.
In particular, the process of U.S. application Ser. No. 218,895 for electrostatic liquid spray application of coatings to a substrate comprises:
The use of supercritical fluids as a transport medium for the manufacture of surface coatings is well known. German patent application 28 53 066 describes the use of a gas in the supercritical state as the fluid medium containing the solid or liquid coating substance in the dissolved form. In particular, the application addresses the coating of porous bodies with a protectant or a reactive or nonreactive decorative finish by immersion of the porous body in the supercritical fluid coupled with a pressure drop to effect the coating. The most significant porous bodies are porous catalysts. However, the applicant characterizes fabrics as porous bodies.
Smith, U.S. Pat. No. 4,582,731, patented Apr. 15, 1986, and U.S. Pat. No. 4,734,451, patented Mar. 29, 1988, describe forming a supercritical solution which includes a supercritical fluid solvent and a dissolved solute of a solid material and spraying the solution to produce a "molecular spray." A "molecular spray" is defined as a spray "of individual molecules (atoms) or very small clusters of the solute." The Smith patents are directed to producing fine films and powders. The films are used as surface coatings.
Coating formulations are commonly applied to a substrate by passing the coating formulation under pressure through an orifice into air in order to form a liquid spray, which impacts the substrate and forms a liquid coating. In the coatings industry, three types of orifice sprays are commonly used; namely, air spray, airless spray, and air-assisted airless spray.
Air spray uses compressed air to break up the liquid coating formulation into droplets and to propel the droplets to the substrate. The most common type of air nozzle mixes the coating formulation and high-velocity air outside of the nozzle to cause atomization. Auxiliary air streams are used to modify the shape of the spray. The coating formulation flows through the liquid orifice in the spray nozzle with relatively little pressure drop. Siphon or pressure feed, usually at pressures less than 18 psi, are used, depending upon the viscosity and quantity of coating formulation to be sprayed.
Airless spray uses a high pressure drop across the orifice to propel the coating formulation through the orifice at high velocity. Upon exiting the orifice, the high-velocity liquid breaks up into droplets and disperses into the air to form a liquid spray. Sufficient momentum remains after atomization to carry the droplets to the substrate. The spray tip is contoured to modify the shape of the liquid spray, which is usually a round or elliptical cone or a flat fan. Turbulence promoters are sometimes inserted into the spray nozzle to aid atomization. Spray pressures typically range from 700 to 5000 psi. The pressure required increases with fluid viscosity.
Air-assisted airless spray combines features of air spray and airless spray. It uses both compressed air and high pressure drop across the orifice to atomize the coating formulation and to shape the liquid spray, typically under milder conditions than each type of atomization is generated by itself. Generally the compressed air pressure and the air flow rate are lower than for air spray. Generally the liquid pressure drop is lower than for airless spray, but higher than for air spray. Liquid spray pressures typically range from 200 to 800 psi. The pressure required increases with fluid viscosity.
Air spray, airless spray, and air-assisted airless spray can also be used with the liquid coating formulation heated or with the air heated or with both heated. Heating reduces the viscosity of the liquid coating formulation and aids atomization.
In general, coating compositions are formulated to help minimize the coating defects that may occur after the coating composition has been sprayed by any of the above means onto the substrate and then dried. Such defects include, but are certainly not limited to, orange peel conditions, runs or sags, pin holing and solvent pops, fish eyes, blistering, and the like, all of which are well known to those skilled in this art.
Indeed, some coating formulations are provided in concentrated form, that is, with a relatively high solids content, so that they may be custom tailored on site by the user. Thus, the user adds an appropriate amount of a particular solvent mixture to accommodate a particular end use spraying condition which may include variable wind conditions, ambient temperatures, drying conditions, humidity, and other such spraying condition factors.
While the above-noted related patent applications all utilize supercritical fluids as a diluent to help reduce the viscosity of highly viscous organic solvent-borne and/or highly viscous non-aqueous dispersions coating compositions so as to facilitate the application of these compositions by liquid spray techniques, and by doing so, desirably reduce the amount of organic solvent Which would otherwise be used, the overall objective, of course, is still to obtain a coated substrate having a uniform, smooth, continuous coating and substantially none of the above-noted defects.
For obvious reasons, none of the prior art coating compositions have been formulated with the intent of having these compositions combined with a supercritical fluid as a diluent and then spraying the resultant admixed liquid mixture through an orifice and onto a substrate to form a liquid coating which is then dried and/or cured.
Indeed, prior to the inventions described in the above-noted related applications and the present invention, it was unknown how a high concentration of highly volatile supercritical fluid, such as supercritical carbon dioxide fluid, would affect formation of a liquid spray containing a solids fraction; a diluent fraction in which said solids fraction is dissolved, suspended or dispersed, and a portion of the supercritical fluid. A spray mixture undergoes a large and rapid drop in pressure as it goes through the orifice. Accordingly, one of ordinary skill in the art could theorize that the supercritical spray mixture would produce a foam like shaving cream instead of a spray, because nucleation to form gas bubbles would be so rapid and intense. Alternatively, one of ordinary skill in the art could also expect that the spray mixture would produce a mist or fog of microdroplets instead of a spray, because atomization would be so intense. Another result that one could theorize is that the spray mixture would produce a spray of bubbles instead of droplets. Furthermore, even if a spray were formed, it would have been expected that the sudden and intense cooling that accompanies rapid depressurization and expansion of a supercritical fluid would cause the liquid droplets to freeze solid. For example, it is commonly known that the spray from carbon dioxide fire extinguishers produces solid dry ice particles.
In the event that formation of a liquid spray were achieved, there is no assurance that the spray could be used to produce quality coherent polymeric coatings on a substrate. One of ordinary skill in the art could surmise that the liquid droplets would be so small or have so little momentum that they could not be deposited well onto the substrate. One could also theorize that foaming droplets or supercritical fluid dissolved in the coating would produce a layer of foam on the substrate or a coating full of bubbles when these characteristics were not desired in the coating. The liquid coating droplets that are deposited onto the substrate would have a much higher viscosity than the material that was sprayed, because they would have lost most of the supercritical fluid diluent and they would be at a lower temperature. Furthermore, the coating material would contain less volatile organic solvent than normal. Therefore, it is not unreasonable to expect that higher viscosity would prevent or hinder coalescence of the deposited droplets to form a coherent liquid coating; that it would reduce how much the droplets spread out on the substrate, so that thin coatings could not be produced; and that it would reduce the surface flow that produces a smooth coating. One can further theorize that moisture would condense onto the droplets and harm the coating, because the spray would be cooled below the dew point.
Surprisingly, however, it has been shown, as discussed in application Ser. No. 883,156 noted above, that liquid sprays can indeed be formed by using supercritical fluids as viscosity reduction diluents and that such sprays can be used to deposit quality coherent polymeric coatings onto substrates.
However, after admixing the highly viscous organic solvent borne and/or highly viscous non-aqueous dispersions coating compositions with supercritical fluids as a diluent to help reduce the viscosity, it may still be desirable to reduce the viscosity even further but keep the overall amount of supercritical fluid used substantially the same. Alternatively, it may also be desirable to maintain (or lower) the viscosity of the admixed coating composition and maintain the overall amount of supercritical fluids used substantially the same, but still want to reduce even further the amount of organic solvent in the admixed coating composition.
More specifically, there may be coating compositions whose initial viscosity is so high that the amount of supercritical fluids that can be admixed with such compositions, without undesirably causing a two phase separation, is insufficient to reduce the viscosity to the point where such composition can properly be sprayed.
Alternatively, since it is known that high molecular weight polymers generally provide finished coatings having better exterior durability, toughness, strength and solvent resistance, it may be desirable to use such a high molecular weight polymer in a coating composition in lieu of a similar but lower molecular weight polymer that may be there. However, the use of such a high molecular weight polymer introduces an increase in the overall viscosity of the coating composition. This increase in viscosity may be such that the amount of supercritical fluids now needed to reduce the viscosity of the composition to a point suitable for spray application may not be obtainable without breaking up the composition into two phases.
Still further, for a given highly viscous coating composition containing a particular amount of polymeric component and an organic or non-aqueous solvent, respectively, it may be desirable to reduce the amount of such volatile solvents even further. Of course, such a reduction in solvent would inherently result in a corresponding increase in the overall viscosity of the coating composition. Here again, the increase in viscosity may be such that the amount of supercritical fluids needed to now reduce the viscosity of the composition to a point suitable for spray application may not be obtainable.
Clearly, a need exists to be able to accomplish all of the above objectives. Preferably, these objectives should be able to be carried out without the necessity of adding supercritical fluid in an amount which is greater than that originally utilized, such that the expected diluent effect of the supercritical fluids can be expected to remain substantially about the same. Of course, if desired, more than the original amount of supercritical fluid may be used, if such amount does not cause the excessive breakup of the composition into two phases.
Accordingly, the present invention provides a means by which the above noted goals may indeed be achieved and, more particularly, provides precursor coating compositions in which those goals have been manifested.
Moreover, a need also exists to provide precursor coating compositions which in addition to achieving the above objectives are also formulated to:
Accordingly, the present invention provides such precursor coating compositions which not only fulfill the goals of (1) having an even lower viscosity and/or (2) having even less organic solvent, but which are also particularly suitable for subsequent admixture with at least one supercritical fluid which admixture is then sprayed through an orifice, such as airless spray or air-assisted airless spray methods, to apply an admixed coating composition onto a substrate which results in a substrate having a substantially uniform, continuous and substantially defect-free coating.